Quantum dot-polymer nanocomposite sensor array for chemical vapor sensing

ABSTRACT

A method of forming a quantum dot-polymer nanocomposite sensing film for chemical vapor sensing is achieved by first obtaining a mass of a type of polymer having a characteristic of expanding or contracting responsive to exposure to at least one of a group of chemical vapors and obtaining a quantum dot (QD) suspension which comprises CDs having a CdSe/ZnS core-shell ratio. The polymer and solvent are mixed, the QD suspension is dried, and the dried QDs are added to the polymer solution and mixed to form a precursor solution. At least a portion of the precursor solution is deposited and spread onto a substrate and then air-dried to form the quantum dot-polymer nanocomposite sensing film.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional patent application of co-pendingapplication Ser. No. 14/804,667, filed on 21 Jul. 2015, now pending. Theentire disclosure of the prior application Ser. No. 14/804,667, isconsidered a part of the disclosure of the accompanying Divisionalapplication and is hereby incorporated by reference.

FIELD OF THE INVENTION

The subject invention relates generally to the field of chemical vaporsensing. In particular, the invention relates to a sensor array forsensing chemical vapors. More specifically, the invention relates toincorporating fluorescent quantum dots into a polymer network to form asensor and utilizing a plurality of sensors to form an array. Each ofthe sensors includes a polymer that is responsive to a particularchemical analyte or group of chemical analytes, and quantum dots whichfluorescence alters when the polymer responds to an analyte, wherebychanges in fluorescence can be measured from all sensors in the arrayand exploited for detecting the presence or a change in concentration ofparticular chemical vapor. A unique combined response pattern from allof the sensors in the array allows the array to distinguish one vaporfrom others, and thus provides detection specificity. Severalalternative arrays and methods for their synthesis, as well as themethod of utilizing the arrays to detect the presence of a particularchemical vapor are presented.

BACKGROUND OF THE INVENTION

The present disclosure relates generally to quantum dot-polymernanocomposites for chemical vapor sensing. In particular, the subjectapplication relates to quantum dot-polymer nanocomposite sensor arraysfor detecting vapors of a defined group of chemical analytes. More inparticular, the subject application relates to the incorporation ofquantum dots into a polymer matrix to form a film wherein the polymer isknown to be responsive to a selective group of chemical vapors. Thereby,when the film is placed into contact with a chemical vapor, interactionbetween functional groups on the polymer and the chemical generatechanges in the polymer network. Expansion or contraction of the polymernetwork responsive to interaction of the polymer and chemical generatemeasurable changes in fluorescence by virtue of the incorporated quantumdots.

Quantum Dots (QDs) are semiconductor nanocrystals which are known fortheir unique size-tunable optical and electronic properties. For thepast few decades, extensive amounts of time, energy, and funding havebeen devoted to research and development efforts exploring the use ofquantum dots (QDs) in a variety of different areas, for example,biological labeling for imaging and monitoring and optical sensing forchemical and biological detection. In the context of chemical andbiological sensing applications, QDs must be functionalized on theirsurfaces, or embedded in a solid state matrix to form a composite. Thisis necessary to avoid QD agglomeration and the consequent fluorescencequenching.

Attributed to their transparence in the ultraviolet-visible (UV-Vis)region of the electromagnetic spectrum, polymeric materials are suitablecandidates to be utilized as matrices for quantum dot composites inoptical sensing applications.

Current fluorescence sensor technologies for chemical and biologicaldetection are based on detecting changes in fluorescence caused byeither a reaction between an organic fluorophore and a target orfluorescence resonance energy transfer (FRET) between two chromophores.The major problem associated with sensing mechanisms which use organicfluorophores is the resultant photo-bleaching and with FRET, it isdifficult to control operation conditions. Neither mechanism is suitablefor use under ambient air conditions.

Therefore, there is a need for a chemical detection sensing system whichis suitable for use in ambient air conditions and which is moresensitive and robust than conventional sensing mechanisms.

By incorporating quantum dots into a polymer network structure to form ananocomposite sensor or a nanocomposite sensor array, the quantum dotsact as optical indicators for optical sensing applications, namely forchemical vapor detection, classification and identification.

SUMMARY OF THE INVENTION

A quantum dot-polymer nanocomposite sensor array for chemical vaporsensing is provided. The quantum dot-polymer nanocomposite sensor arraycomprises an array of sensors for detecting vapors of any of a definedgroup of chemical analytes. The array of sensors includes a plurality ofsensors respectively formed by different nanocomposite films. Each ofthe nanocomposite films includes a different polymer that iscorrespondingly responsive to at least one of the chemical analytes andfluorescent QDs. The plurality of sensors are disposed on a substratefor impingement by an excitation light source. Responsive to exposure toa vapor of any one of the chemical analytes, the combined fluorescenceresponse of the plurality of sensors uniquely identifies the chemicalanalyte.

In another aspect, a method of forming a quantum dot-polymernanocomposite sensing film for chemical vapor sensing is provided.First, a mass of a type of polymer known to be responsive to a selectivegroup of chemical vapors is obtained and a quantum dot (QD) suspensionis obtained. Then, the polymer is mixed with a solvent until a clearpolymer solution results and the QD suspension is dried until thesuspension becomes wax-like. Next, the dried QDs are added to thepolymer solution and thereafter, the dried QD and polymer solution aremixed to form the precursor solution. At least a portion of theprecursor solution is then deposited onto a substrate and the mixture isspread thereon. Finally, the precursor solution is air-dried to form thequantum dot-polymer nanocomposite sensing film.

From another aspect, a method of detecting the presence of a chemicalvapor of interest begins with manufacturing a plurality of quantum dotpolymer nanocomposite sensing films wherein each of the sensing filmscomprises a plurality of quantum dots (QDs) doped into a polymer networkto form a film wherein the polymer network is known to be responsive toa selective group of chemical vapors. The quantum dot polymernanocomposite films are impinged by an excitation light source and thenthe sensing films are exposed to a suspected contaminant. Next, thechange in fluorescence emission intensity from the films is detectedthat is responsive to the excitation light source over time and thesuspected contaminant. Thereafter, the changes in fluorescence emissionintensity from the films are compared to establish a unique combinedresponse.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of the change in spacing of aQD-polymer network responsive to the presence of a particular chemicalanalyte.

FIG. 2 is a schematic illustration of an operational test device fortesting the responsiveness of sensors to chemical analytes.

FIG. 3 is a schematic illustration of an optical detection system fordetecting the presence of a chemical vapor of interest using separateexcitation light sources.

FIG. 4 is a schematic illustration of an optical detection system fordetecting the presence of a chemical vapor of interest using a commonexcitation light source.

FIG. 4A is a schematic illustration of a sensor substrate with sensorsdisposed thereon to be used in conjunction with the optical detectionsystem shown in FIG. 4.

FIG. 5 is a cross sectional view of a probe used in conjunction with theoptical detection systems shown in FIGS. 3 and 4.

FIG. 6A is a graphical representation of the fluorescence spectra of aQD/PEG sensor film before and after exposure to acetone.

FIG. 6B is a fluorescence microscope image showing a QD/PEG sensor filmbefore and after exposure to acetone.

FIG. 7 is a bar graph showing normalized responses of a six sensor arrayto 9 different chemical vapors.

FIG. 8 is a bar graph showing normalized responses of a ten sensor arrayto 8 different chemical vapors.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the terms and phrases used herein have their art-recognizedmeaning, which can be found by reference to standard texts, journalreferences, and context known to those skilled in the art. The phrase“target chemical analytes” denotes chemical analytes to which at least asubset of sensors in a given quantum dot-polymer nanocomposite sensorarray will respond.

The following details are intended to explain the inventive concept ofthe subject Patent Application. However, such are provided forillustrative purposes and not intended to limit the scope of theinvention. It will be apparent to those skilled in the art that a numberof substitutions of, for example, different quantum dots, polymers andchemical compounds may be made without departing from the scope of theinvention.

In general concept, the subject Application is directed to a quantumdot-polymer nanocomposite sensor array for chemical vapor sensing, amethod of forming the quantum dot-polymer nanocomposite sensor array anda method of detecting the presence of a chemical vapor of interest.

The quantum dot-polymer nanocomposite sensor array comprises an array ofsensors suitable for detecting vapors of a defined group of chemicalanalytes. The plurality of sensors that form the array are made of ananocomposite film wherein each film includes a polymer which isresponsive to at least one chemical analyte of interest.

The sensors are disposed on a substrate for impingement by an excitationlight source. When the sensors are exposed to one of the chemicalanalytes of the group, one of three things happens to the polymernetwork: it expands, it contracts or it remains the same. FIG. 1provides a schematic illustration of the change in spacing, in otherwords, the expansion or contraction of a QD-polymer network responsiveto the presence of a particular chemical vapor. Expansion of theQD-polymer network is associated with a corresponding increase influorescence intensity while contraction of the network brings about adecrease in fluorescence intensity. Maintenance of the same level offluorescence indicates that the polymer network did not expand orcontract in response to the chemical vapor with which it was placed incontact.

An exemplary operational test device (also referred to herein as a testsystem) used for testing the responsiveness of sensors to chemicalanalytes is provided in FIG. 2. Such a test system includes generally anair sampler tube 14, a probe 18, an LED light source 22 and aspectrometer 20. The LED light source 22 and spectrometer 20 areconnected to the probe 18, respectively, by optical fibers 24 a and 24b. A sensing film (which includes substrate 16 and sensors 15) isdisposed between the air sampler tube 14 inlet and the probe 18, and afan 17 is disposed beneath the probe 18 to circulate air through thesystem. For instance, a sample 12 containing a target chemical analyteis disposed above the air sampler tube 14 inlet and the fan 17 causesair to flow through the device in a predetermined direction shown bydirectional arrows 13. The fan 17 brings the target chemical analyte ofthe sample 12 to come into contact with the sensing film to enable thesystem to determine whether a target chemical analyte is present in thesample 12. Alternative devices for effectuating air flow through thesystem may be employed such as, for example, a blower disposed externalto the air sampler tube 14 to blow the sample 12 into air sampler tube14.

After the sample 12 is brought into contact with the sensing film,excitation light is transmitted from the LED light source 22 to theprobe 18 through optical fiber 24 a. The substrate 16 is transparent orat least light transmissive. For example, the substrate 16 may be atransparent plastic film, a glass slide, or a top surface of the glassfiber of the probe 18, whereby sensors 15 on the upper surface of thesensor substrate 16 are excited by light from LED light source 22 ofprobe 18 through the substrate 16. When the glass fiber is thesubstrate, a different sensor is deposited on each fiber and theillumination source is provided external to the probe. In response,emission light collected from the sensor substrate 16 is transmitted tothe spectrometer 20 through optical fiber 24 b. A computer (not shown inFIG. 2) interfaces with the spectrometer 20 to retrieve the signal andprocess the data.

The components of the test device are commercially available productsand are shown herein for exemplary purposes only. Suitable alternativesmay be made for the individual components shown in FIG. 2.

Utilizing such systems, a sensor response is obtained for each sensor ofthe array. The response is based upon fluorescence changes before andafter exposure to a particular chemical. The signal detector reads thefluorescence signals that are subsequently processed to obtain the ratesof change for comparison. The responses are categorized as “positive,”“negative” or “zero.” A polymer network expansion in the presence of aparticular chemical vapor causes a corresponding increase influorescence which is a “positive” response while a polymer networkcontraction in the presence of a chemical vapor causes a correspondingdecrease in fluorescence which is a “negative” response. A “zero”response is one which is at noise level and can therefore be neglected.FIGS. 7 and 8 respectively show the responsiveness of a six sensor arrayand a ten sensor array to a variety of different chemical analytes.

Though certain polymers are discussed herein with respect to theexemplary embodiments, there are a multitude of additional polymers thatare useful for forming a sensor array. Suitable polymer candidates donot absorb the excitation light of the associated quantum dots and theyare soluble or able to swell in a certain solvent. The polymer solutionmust also be capable of being deposited on a substrate to form a filmand must exhibit reactivity to certain target analytes.

Design of the sensors of the QD-polymer nanocomposite sensor array isbased upon the reactivity of the functional groups associated with andthe polarity of the polymer along with the size of the target analyte.The reactivity of the functional group on the polymer used determines towhich functional group it will respond on the target chemical ofinterest.

Polarity of the polymers can be used to discriminate between hydrophilicand hydrophobic analytes. Polarity is also useful for differentiatingchemical analytes of interest when those chemical targets have the samefunctional group, for instance, targets which simply differ in thenumber of carbon backbones that they possess. Molecular weight of thepolymers can also be utilized as a parameter for designing theindividual sensors making up the array. For instance, a polymer having arelatively lower molecular weight will be able to react much morequickly with smaller molecular analytes.

TABLE 1.1 lists a number of common polymers along with a representativesampling of target chemical vapors with which the polymers will react toelicit a measurable change.

TABLE 1.1 Polymer Target Chemical Vapors Polystyrene aromatichydrocarbons (e.g., benzene, toluene, xylene, ethylbenzene); chlorinatedaliphatic hydrocarbons (e.g., methylene chloride, chloroform, carbontetrachloride); ketones (e.g., acetone, methyl ethyl ketone, diisopropylketone); cyclohexane, alkyl-cyclohexanes (e.g., methylcyclohexane,ethylcyclohexane); esters (e.g., ethyl acetate, butyl acetate);pyridine; dioxane; dimethylformamide; cyclohexanone; tetrahydrofuran;n-butyl phthalate; methyl phthalate;, ethyl phthalate;tetrahydrofurfuryl alcohol; 1-nitro-propane; carbon disulfide andtributyl phosphate Poly (para- aromatic hydrocarbons (e.g., benzene,toluene, -substituted styrene), xylene); chloroform; carbontetrachloride; including poly- dichloromethane (DCM); dichloroethane(DCE; 4-methoxystyrene, and tetrahydrofuran (THF) poly-4-methylstyrene,poly-α-methylstyrene, poly-4-tert- butylstyrene, poly-4- chlorostyrene,poly-4- bromostyrene Polystyrene-co-allyl ketones (e.g. methyl ethylketone); toluene; acids alcohol (acetic acid) and tetrahydrofuranPolystyrene-co- aromatic hydrocarbons (e.g., benzene, toluene, butadienexylene); tetrahydrofuran; alkane (e.g., hexane) Poly-2-hydroxyethylwater; alcohols and acids (e.g., acetic acid) methacrylate Polymethyl-alcohols (e.g., methanol, ethanol); ketones (e.g., methacrylateacetone);, esters (e.g., methyl acetate, ethyl acetate); chlorinatedsolvents (e.g., dichloromethane, trichloromethane, chloroform) and acids(e.g., acetic acid) poly-4-vinylphenol alcohols (e.g., methanol,ethanol, 2-propyl alcohol, t-butyl alcohol); ketones (e.g., acetone);acetonitrile; dioxane; THF and DMSO Poly-4-vinylphenol- alcohols (e.g.,methanol, ethanol, , 2-propyl co-methyl alcohol, t-butyl alcohol);ketones (e.g., acetone); methacrylate esters (e.g., methyl acetate,ethyl acetate);, chlorinated solvents (e.g., dichloromethane,trichloromethane, chloroform) and acids (e.g., acetic acid)polyethyleneimine Water Hydroxypropyl water; alcohols and anhydroussolvents (e.g., polar cellulose organic solvents and glycols)Polybisphenol- chlorinated solvents (e.g., carbon tetrachloride orA-carbonate methylene chloride) Polysulfone chlorinated hydrocarbons(e.g., carbon tetrachloride or dichloromethane) Polyvinylidene dimethylsulfoxide or tetrahydrofuran fluoride Polycaprolactone Tetrahydrofuranand chloroform, esters (e.g. ethylacetate) polyethylene glycol Water;alcohols, ketones (e.g, acetone) and acids (e.g., acetic acid) Poly-N-Water; polar solvents including alcohols, ketones vinylpyrrolidone(e.g., acetone) and acids (e.g., acetic acid) polyethylene-co-Tetrahydrofuran; aromatic hydrocarbons (e.g., vinyl acetate benzene,toluene and xylene);, cyclohexane and chlorinated solvents (e.g.,chloroform and o- dichlorobenzene)

In certain exemplary embodiments, the sensor array is designed such thatthe quantum dots of each sensing film have the same emission wavelength.In such instances, each sensor/sensing film is individually excitablethereby enabling separate and independent detection of the changes influorescence emission intensity of each sensor. FIG. 3 provides anexample of the set up for exciting each of the sensing films by anindependent excitation light source. As shown in FIG. 3, each sensorsubstrate 16 has a sensor 15 a-15 f disposed thereon to form a sensingfilm. Each sensing film is associated with an independent probe 18 andan independent detection system 126 which includes a signal detector(a-f) and an excitation source (a-f). Each excitation source (a-f) is anLED light source and each signal detector (a-f) is a spectrometer whichare connected to the respective probe via optical fibers.

Each probe 18 includes an optical fiber for providing excitation light(not shown for simplicity) which is different for each of the sixsensors. Each probe 18 also has a plurality of optical fibers forproviding emission light wherein the emission light associated with eachof the plurality of optical fibers is the same. FIG. 5 is across-sectional view of a commercially-available probe used inconjunction with the systems depicted in FIGS. 3 and 4. These probes 18are optical components which include a fiber 18 a to bring excitationlight to the sensing film and another fiber (or fibers) 18 b to collectemission light from the sensing film. The use of multiple fibers tocollect light is for the purpose of signal amplification.

As shown in FIG. 3, emission and excitation light rays 50 are exchangedbetween the probes 18 and sensing films. An excitation source (a-f) ofdetection system 126 transmits excitation light, via the optical fiberfor excitation light 18 a (shown in FIG. 5), such that excitation lightpasses through substrate 16 to excite the respective sensor 15 (a-f).The emission light is collected by and transmitted through opticalfibers for emission light 18 b (shown in FIG. 5) and a signal detector(a-f) of detection system 126 measures the fluorescence signal which issubsequently processed by the data processor 40 to determine the rate ofchange. The sensor response to a target analyte results in a change inpolymer network structure. The later causes a light scattering effectthat alters the detected QD fluorescence. The change in fluorescenceintensity and/or rate of change is measured to detect the presence of atarget analyte.

Six sensor films including sensors 15 a-15 f and their correspondingsensor substrates 16 are depicted in FIG. 3, however such is providedfor exemplary purposes only and it should be appreciated that the numberof sensors may vary depending upon the particular application andobjectives.

In certain alternative embodiments, the sensor array is designed suchthat each sensor is made up of quantum dots having emission wavelengthsdifferent from the other sensors' emission wavelengths. In suchinstances, all of the sensors/sensing films are excited by a commonexcitation light source 122 as shown in FIG. 4. FIG. 4 provides anexample of the set up for a system where the quantum dots associatedwith each of the six sensors are unique from the quantum dots associatedwith the other sensors. In that case, the six sensors 15 a-15 f aredisposed on a common substrate 16 as shown in FIG. 4A. As shown in FIG.4, when the quantum dots of the various sensors each have a differentemission wavelength, a single probe 18 fed by an optical fiber is usedto impinge excitation light onto the sensors 15 a-15 f disposed onsubstrate 16. The probe 18 also has a plurality of optical fibers forreceiving and transmitting emission light to a detector 26 (aspectrometer) which reads the emission intensities of different QDfluorescence at different wavelengths. In such cases, there must beenough of a difference between the individual wavelengths of the QDs toavoid interference. The spectrometer scans a wavelength range of, forexample, 480 nm-700 nm.

As shown in FIG. 4, emission and excitation light rays 50 are exchangedbetween the probe 18 and sensing films. An excitation source 122transmits excitation light, via the optical fiber for excitation light18 a (shown in FIG. 5), such that excitation light passes throughsubstrate 16 to excite the respective sensors 15 (a-f). The emissionlight is collected by and transmitted through optical fibers foremission light 18 b (shown in FIG. 5) and a signal detector 26 measuresthe fluorescence signal which is subsequently processed by the dataprocessor 40 to determine the rate of change. The sensing film with asingle substrate 16 and six sensors 15 a-15 f shown in FIGS. 4 and 4A isprovided for exemplary purposes only. The number of sensors utilized mayvary depending upon the particular application and objectives.

When the sensor is placed in the presence of a chemical vapor, reactionbetween the polymer and the chemical will cause the polymer network toexpand or contract. When impinged by an excitation light, an expandedpolymer network will result in a correspondingly stronger fluorescentemission. On the other hand, contraction of the polymer network willgenerate a weaker fluorescent emission. Such expansion/contraction andthe corresponding fluorescent emission intensity is shown in FIG. 1.

The sensor array can be applied to a wide variety of sensingapplications. For instance, in an embodiment the sensor arrays can beused for chemical vapor detection, classification, and identification.The array is capable of discriminating different chemical vapors and canthus classify and identify chemical vapors that may be noxious. Thearray is therefore helpful in applications designed to detect toxicchemical contamination in air or those designed to warn of a chemicalleak from a sealed container.

Additionally, in another embodiment the sensor array can be utilized forthe detection of biological agents via sensing their volatilemetabolites. Viable biological agents produce certain metabolites. E.coli cells, for example, produce indole which is a volatile organiccompound. The sensor array can be designed to detect indole andtherefore serve as a useful tool for screening for the presence of E.coli bacteria.

Furthermore, other embodiments encompass the sensor array applied todisease diagnosis via detection of particular human odor markers.Patients with certain diseases are known to have higher concentrationsof certain volatile organic compounds present in their bodily fluids.Therefore, by testing a patient's breath, sweat, or urine, for example,the sensor array can be utilized for disease detection. By way ofexample, higher concentrations of acetone and methylethylketone,n-propanol, to name a few, are detectable in the breath of patients withlung carcinoma. The sensor array can be utilized to detect the targetcompounds for screening such medical conditions and therefore serve as auseful tool for early detection and treatment.

Example 1

In one working embodiment, a QD-polymer nanocomposite sensor array ismade up of six sensors. Each of the sensors includes a plurality ofquantum dots and a different polymer corresponding to each of the sixsensors. Sensor 1 includes quantum dots doped in a PMMA polymer whilesensor 2 includes quantum dots doped in a PVP polymer. Sensor 3 includesquantum dots doped in a PEG polymer and Sensor 4 includes quantum dotsdoped in a Polycaprolactone polymer. Sensor 5 includes quantum dotsdoped in a HPC polymer and Sensor 6 includes quantum dots doped in aPolystyrene co allyl alcohol polymer.

Each sensor of the array is formed by a different nanocomposite film.Each film was created by first obtaining a mass of each of the sixdifferent polymers (PMMA, PVP, PEG, Polycaprolactone polymer, HPC andPolystyrene co allyl alcohol) and a QD suspension. To form the QDsuspension, QDs having a CdSe/ZnS core-shell ratio were purchased fromSigma Aldrich (product #694622) and the QDs were stabilized withHexadecylamine (HAD) ligand surface coating to have a fluorescenceemission wavelength of 590 nm. Each of the six polymers were mixed witha specific solvent as set forth in TABLE 2.1.

TABLE 2.1 Sensor Polymer Solvent 1 PMMA Toluene 2 PVP Water 3 PEG Water4 Polycaprolactone Tetrahydrofuran 5 HPC Water 6 Polystyrene co allylalcohol Tetrahydrofuran

While TABLE 2.1 provides for using one particular solvent correspondingto each of the 6 polymers, it will be apparent to those skilled in theart that alternative solvents having the same or similar chemicalproperties may be utilized so long as the polymer is capable ofdissolving in the solvent chosen. For instance, rather than using waterwith the PVP and PEG of Sensors 2 and 3, ethanol could be used.Similarly, for sensor 4, rather than using tetrahydrofuran one maysubstitute chloroform or ethylacetate. For sensor 5, water was mixedwith HPC but alcohol and anhydrous systems, for instance, polar organicsolvents and glycols are acceptable alternatives.

In some instances, the polymer may swell when it comes into contact withthe solvent. If swelling occurs, the mixture is given a period of timeto sit and allow the swelling to subside. For example, the mixture maybe allowed to sit overnight or for an equivalent period of time, such asapproximately eight hours. In designing the sensors of the six sensorarray, mixing was accomplished by vortexing for five minutes until aclear polymer solution was obtained. Alternative methods of mixing suchas shaking and sonicating can also be employed. The length of time formixing may vary dependent upon the combination of polymer and solventused but, regardless of the method of mixing used, mixing should becarried out until the color of the mixture is homogenous.

Each of the 6 polymers and solvents were mixed in the ratio of 1.0 gpolymer to 40 ml solvent until a clear polymer solution resulted. Then,the QD suspension having a concentration of 5 mg/ml was dried until thesolution adopted a wax-like appearance. Thereafter, the dried QDs wereadded to each of the polymer solutions and were mixed to form precursorsolutions. A portion of each precursor solution was deposited onto asubstrate (in this case a glass slide was used) and spread thereupon.Finally, the precursor solutions were air-dried to form quantumdot-polymer nanocomposite sensing films. The amount of time required todry the suspension is dependent upon the type of polymer and solventsused. For instance, PEG in ethanol may dry after a few minutes while PEGin water takes approximately one hour.

The 6 sensor array was utilized to determine combined responses to 9different chemical vapors, namely, acetic acid, acetone, ethyl acetate,chlorobenzene, methanol, ethanol, isopropanol, toluene and xylene. Sinceeach of the sensor films were created using QDs with 590 nm fluorescenceemission wavelength, the detection system utilized 6 probes—each probedesigned to individually excite the sensor films allowing forindependent detection of changes in fluorescence emission intensity ofeach sensor. A schematic of the optical detection system employed isshown in FIG. 3. As seen in FIG. 3 each of the six sensor films isassociated with a unique probe 18 and detection system 126 and theinformation obtained is detected by a common data processor 40 aspreviously discussed herein.

To test the sensor array response to chemical vapors, the array wasfirst exposed to ambient air for 10 minutes. The length of time duringwhich the array is exposed to ambient conditions may be adjusted basedupon alternative experimental conditions such as, for example, use ofdifferent polymers. During exposure to ambient air, the fluorescenceintensity of each sensor of the array was measured every 30 seconds.Thereafter, the array was exposed to one of the 9 different chemicals,namely, acetic acid, acetone, ethyl acetate, chlorobenzene, methanol,ethanol, isopropanol, toluene and xylene, and fluorescence was measuredevery 30 seconds. While measurements were taken in 30 second increments,it will be appreciated that alternative sampling rates may be employed.The optimal data sampling rate is dependent upon the particularapplication and may be adjusted accordingly.

FIG. 6A provides a graphical representation of the fluorescence spectraof a QD/PEG sensor (sensor 3) before and after exposure to acetone. Thelower peak and higher peak represent fluorescent intensity before andafter exposure to acetone, respectively. Such fluorescence is alsoevidenced in FIG. 6B which is a fluorescence microscope image showing aQD/PEG sensor film before and after exposure to acetone. The increase influorescent intensity of the QD/PEG sensor indicates that the polymernetwork expands in the presence of acetone.

The data shown in FIG. 6A is representative of the data obtained fromeach measurement (raw data). This data is used for data processingwherein the first step is to integrate the peak to obtain emissionintensity for each data point (each measurement). The change rate isthen calculated using three consecutive data points.

Three data points prior to chemical exposure were used to calculate thepre-exposure signal change rate (r1) and three data points afterchemical exposure were used to calculate the post-exposure signal changerate (r2). This process was repeated until data was obtained for each ofthe 9 chemical vapors set forth in TABLE 2.1. The difference in thesignal change rate prior to and after chemical vapor exposure (r2−r1)was calculated for each sensor and then normalized using the highestdifference as 100 or −100. The normalized response for each independentsensor of the array to the 9 chemical vapors tested is reflected inTABLE 2.2 and graphically depicted in FIG. 7.

TABLE 2.2 Polymer Used in Acetic Ethyl Each Sensor Acid Acetone AcetateChlorobenzene Methanol Ethanol Isopropanol Toluene Xylene Sensor 1: PMMA−43.95 1.01 −2.08 −1.08 −23.10 −13.10 −3.10 3.94 4.68 Sensor 2: PVP−97.50 11.91 5.09 −100.00 65.00 12.00 4.00 20.54 12.58 Sensor 3: PEG−20.44 100.00 7.79 2.79 25.61 −22.61 −12.61 12.20 3.58 Sensor 4: −50.4052.06 −7.99 −3.99 98.15 93.15 3.15 100.00 35.55 Polycaprolactone Sensor5: HPC −100.00 42.29 −2.09 −5.09 −100.00 −100.00 −1.06 18.49 −100.79Sensor 6: 7.68 −6.66 100.00 −1.90 −23.57 3.57 100.00 −10.58 −0.29Polystyrene co allyl alcohol

As can be seen in TABLE 2.2, sensor 4 elicited a normalized responsewhich was a high, “positive response” (98.15) to methanol. This positiveresponse, which can be seen with reference to FIG. 7, shows that therewas an increase in fluorescence when sensor 4 was placed in the presenceof the chemical of interest, namely, methanol Such a positive responseindicates that the polycaprolactone polymer network of sensor 4 expandedin the presence of methanol. In contrast, sensor 5 elicited a normalizedresponse which was a low, “negative response” (−100.00) to methanol.This negative response, which can also be seen with reference to FIG. 7,shows that there was a decrease in fluorescence when sensor 5 was placedin the presence of methanol Such a negative response indicates that theHPC polymer network of sensor 5 contracted in the presence of methanol.Finally, a positive or negative response may be obtained which is closeto zero, for instance, the response of sensor 1 to acetone (1.01) andethyl acetate (−2.08). Such minor changes in fluorescence are at noiselevel and are considered negligible. Therefore, the measured responsesof sensor 1 to acetone and ethyl acetate indicate that the PMMA polymernetwork of sensor 1 neither expanded nor contracted in the presence ofacetone or ethyl acetate.

Then, using the normalized response from each sensor as an element, thecombined response from all 6 sensors can be expressed by a vector. Forinstance, the combined response for acetic acid is [−43.95, −97.50,−20.44, −50.40, −100.00, 7.68].

Example 2

In another working embodiment, a QD-polymer nanocomposite sensor arrayincludes ten sensors. Each of the sensors includes a plurality ofquantum dots and a different polymer. Sensors 1-5 include quantum dotsdoped into the same polymers as specified for Sensors 1-5 in the 6sensor array above. In addition, sensor 6 includes quantum dots doped ina poly (allylamine hydrochloride) polymer and sensor 7 includes quantumdots doped in a poly(carbonate bisphenol A) polymer. Sensors 8, 9 and 10include quantum dots respectively doped in a poly(ethylene-co-vinylacetate) polymer, a polysulfone polymer, and a poly(sodium4-styrenesulfonate) polymer.

Each sensor of the array is formed by a different nanocomposite film andeach film was created by the methodology outlined with respect toEXAMPLE 1. The first five sensors of the ten sensor array are the sameas those in the six sensor array previously described herein. The tenpolymers were respectively mixed with the solvents as set forth in TABLE3.1.

TABLE 3.1 Sensor Polymer Solvent 1 PMMA Toluene 2 PVP Water 3 PEG Water4 Polycaprolactone Tetrahydrofuran 5 HPC Water 6 Poly(allylaminehydrochloride) Water 7 Poly(carbonate bisphenol A) Carbon tetrachloride8 Poly(ethylene-c-vinyl acetate) Tetrahydrofuran 9 Polysulfone Carbontetrachloride 10  Poly(sodium 4-styrenesulfonate) Water

While TABLE 3.1 provides for using one particular solvent correspondingto each of the 10 polymers, it will be apparent to those skilled in theart that alternative solvents may be utilized. For example, the samesubstitutions for sensors 2, 3 and 4 can be made as previously discussedherein with regard to EXAMPLE 1. For sensor 7, Poly(carbonate bisphenolA) may be mixed with a variety of different chlorinated solvents such asmethylene chloride in place of carbon tetrachloride. Similarly, thepolysulfone polymer of sensor 9 may be mixed with tetrahydrofuran,dimethyl sulfoxide or another hexane chlorinated hydrocarbon such asdichloromethane in place of the carbon tetrachloride which was used inthe 10 sensor array.

The 10 sensor array was tested to determine combined responses to 8different chemical vapors, namely, 2-methylpentane, acetic acid,acetone, ethanol, indole, pentane, toluene and water. As in EXAMPLE 1,combined responses were obtained by first exposing the array to ambientair for 10 minutes. During exposure to ambient air, the fluorescenceintensity of each sensor of the array was measured every 30 seconds.Thereafter, the array was exposed to one of the 8 different chemicals,namely, 2-methylpentane, acetic acid, acetone, ethanol, indole, pentane,toluene and water, and fluorescence was measured every 30 seconds. Threedata points prior to chemical exposure were used to calculate thepre-exposure signal change rate (r1) and three data points afterchemical exposure were used to calculate the post-exposure signal changerate (r2). This process was repeated until data was obtained for each ofthe 8 chemical vapors set forth in TABLE 3.1.

The difference in the signal change rate prior to and after chemicalvapor exposure (r2−r1) was calculated for each sensor and thennormalized using the highest difference as 100 or −100. The normalizedresponse for each independent sensor of the array to the 8 chemicalvapors tested is reflected in TABLE 3.2 and graphically depicted in FIG.8.

TABLE 3.2 Polymer Used 2-Methyl- Acetic in Each Sensor pentane AcidAcetone Ethanol Indole Pentane Toluene Water Sensor 1: 0.13 −25.33 2.01−7.38 −23.21 −100.00 3.94 1.40 PMMA Sensor 2: PVP 8.65 −52.76 14.14 6.76−39.33 32.21 20.54 11.13 Sensor 3: PEG −6.69 −9.85 100.0 −12.73 42.79−4.32 12.20 −100.00 Sensor 4: 9.47 −26.76 55.06 52.44 44.77 24.07 100.006.37 Polycaprolactone Sensor 5: HPC 4.23 −53.09 43.29 −56.30 9.61 −29.8218.49 3.42 Sensor 6: 12.61 −4.81 49.79 −23.06 42.53 34.17 −0.35 15.73poly (allylamine hydrochloride) Sensor 7: −12.65 −100.00 69.89 100.0044.77 49.25 16.78 −4.75 poly (carbonate bisphenol A) Sensor 8: −30.4929.69 45.80 −0.17 100.00 4.89 23.66 18.45 poly (ethylene- co-vinylacetate) Sensor 9: 100.00 −49.67 8.34 −21.30 −12.33 84.68 34.24 13-21polysulfone Sensor 10: 4.84 −5.73 6.42 −13.17 18.31 7.43 0.47 −11.14poly (sodium 4- styrenesulfonate)

As can be seen in TABLE 3.2, sensor 7 elicited a normalized responsewhich was a high, “positive response” (100.00) to ethanol. This positiveresponse, which can be seen with reference to FIG. 8, shows that therewas an increase in fluorescence when sensor 7 was placed in the presenceof the chemical of interest, namely, ethanol Such a positive responseindicates that the poly(carbonate bisphenol A) polymer network of sensor7 expanded in the presence of ethanol. In contrast, sensor 7 elicited anormalized response which was a low, “negative response” (−100.00) toacetic acid. This negative response, which can also be seen withreference to FIG. 8, shows that there was a decrease in fluorescencewhen sensor 7 was placed in the presence of acetic acid. Such a negativeresponse indicates that the poly(carbonate bisphenol A) polymer networkof sensor 7 contracted in the presence of acetic acid. Finally, apositive or negative response may be obtained which is close to zero,for instance, the response of sensor 10 (0.47) and the response ofsensor 6 (−0.35) to toluene. Such minor changes in fluorescence are atnoise level and are considered negligible. Therefore, the measuredresponses of sensors 6 and 10 to toluene indicate that thepoly(allylamine hydrochloride)polymer of sensor 6 and the poly(sodium4-styrenesulfonate) polymer of sensor 10 neither expanded nor contractedin the presence of toluene.

Then, using the normalized response from each sensor as an element, thecombined response from all 10 sensors can be expressed by a vector. Forinstance, the combined response for 2-Methylpentane is [0.13, 8.65,−6.69, 9.47, 4.23, 12.61, −12.65, −30.49, 100.00, 4.48].

The descriptions above are intended to illustrate possibleimplementations of the present invention and are not restrictive. Whilethis disclosure has been made in connection with specific forms andembodiments thereof, it will be appreciated that various modificationsother than those discussed above may be resorted to without departingfrom the spirit or scope of the claimed invention. Such variations,modifications, and alternatives will become apparent to the skilledartisan upon review of the disclosure. For example, functionallyequivalent elements or method steps may be substituted for thosespecifically shown and described, and certain features may be usedindependently of other features, and in certain cases, particularlocations of elements or sequence of method steps may be reversed orinterposed, all without departing from the spirit or scope of theinvention as defined in the appended Claims. The scope of the claimedinvention should therefore be determined with reference to thedescription above and the appended claims, along with their full rangeof equivalents.

What is claimed is:
 1. A method of forming a quantum dot-polymernanocomposite sensing film for chemical vapor sensing comprising: (a)obtaining a mass of a type of polymer having a characteristic ofexpanding or contracting responsive to exposure to at least one of agroup of chemical vapors and obtaining a quantum dot (QD) suspension,the QD suspension comprising QDs having a CdSe/ZnS core-shell ratio; (b)mixing the polymer with a solvent until a clear polymer solutionresults; (c) drying the QD suspension until the suspension becomeswax-like; (d) adding the dried QDs to the polymer solution; (e) mixingthe dried QDs and polymer solution to form the precursor solution; (f)depositing at least a portion of the precursor solution onto a substrateand spreading the mixture thereon; and (g) air-drying the precursorsolution to form the quantum dot-polymer nanocomposite sensing film. 2.The method of forming a quantum dot-polymer nanocomposite sensing filmas claimed in claim 1, wherein in step (b), the polymer and solvent aremixed in a ratio of 1.0 g polymer to 40 ml solvent.
 3. The method offorming a quantum dot-polymer nanocomposite sensing film as claimed inclaim 1, wherein in step (b), the polymer and solvent are mixed byvortex or sonication.
 4. The method of forming a quantum dot-polymernanocomposite sensing film as claimed in claim 1, wherein in step (a)the concentration of the QD suspension is 5 mg/ml.
 5. The method offorming a quantum for-polymer nanocomposite sensing film as claimed inclaim 1, wherein in step (d) the dried QD are added to the polymersolution in a ratio of 1 mg dried QD to 0.40 ml polymer solution.
 6. Themethod of forming a quantum dot-polymer nanocomposite sensing film asclaimed in claim 1, wherein in step (e) the dried QD and polymersolution are mixed by vortexing for 5 minutes.
 7. The method of forminga quantum dot-polymer nanocomposite sensing film as claimed in claim 1,wherein if the polymer swells in the solvent in step (b), the mixture isgiven eight hours for the swelling to subside.